1. Field of the Invention
The present invention relates to a lighting control circuit for vehicle lamps, and particularly relates to a lighting control circuit for vehicle lamps that controls the lighting of a semiconductor light source constituted by a semiconductor light-emitting device.
2. Related Art
There is a vehicle lamp wherein a semiconductor light-emitting device such as LEDs (Light Emitting Diodes) is employed as light sources. A lighting control circuit for controlling the lighting of LEDs is mounted on the vehicle lamp.
In the lighting control circuit, one type of a lighting control circuit employs a switching regulator for controlling voltages to be output to LEDs based on the current supplied to the LEDs. Even when a plurality of LEDs are connected in series or in parallel to the switching regulator, the switching regulator can control the output voltage so that a designated current can be supplied to the individual LEDs.
However, when the output of the switching regulator is short-circuited, or is disabled by a ground-fault, an increased load will be imposed on the switching regulator, which will malfunction if the power load becomes excessive. Furthermore, if the output side of the switching regulator is opened due to a disconnection, the output voltage will be sharply raised when the switching regulator is, for example, a flyback type.
Therefore, JP-A-2004-134147 discloses a switching regulator in which an output voltage is dropped when an abnormality is detected on an output side of the switching regulator.
In the lamp of JP-A-2004-134147, a plurality of LED blocks with a plurality of LEDs connected in series are connected in parallel on the output side of a switching regulator. The individual LED blocks and the switching regulator are connected via a shunt resistor, and the output voltage is controlled so that a predetermined current flows through the shunt resistor, and so that the output voltage is dropped when an abnormality occurs. Thus, even when a failure affecting part of the LEDs occurs, the rest of the LEDs are protected and in a normal state. However, this arrangement cannot always satisfactorily supply a predetermined current to the individual LED blocks. That is, since the current supplied to each LED block is not monitored, this arrangement cannot adequately and consistently supply a predetermined current to all the LED blocks, even when all the LED blocks are in the normal state.